Motor vehicle transmissions designed with a group configuration are used in particular in the sector of commercial vehicles and enable the engagement of a large number of shiftable gears. Often, a motor vehicle transmission of this type consists of a main group, a splitter group connected upstream therefrom and a range group usually connected downstream therefrom, while depending on the application further transmission groups can also be provided, such as a reversing group and a crawling group.
By means of a main group, gear steps of the transmission are defined, which are influenced correspondingly by the other transmission groups connected upstream and downstream therefrom. Thus, an upstream splitter group compresses a gear sequence of the main group in that the gear steps of the main group are separated by small step intervals of the splitter group. In contrast, the downstream range group extends the gear sequence of the main group by virtue of transmission ratio intervals that are chosen large, whereby the gear steps of the main group are distributed between different gear ratio ranges.
Whereas in such cases a splitter group and even a main group of a motor vehicle transmission designed with a group configuration are often designed to be powershiftable, a range group connected downstream is in most cases still designed as a shiftable, variable-speed transmission unit in which a change between the different range ratios is only possible with interruption of the traction force. However, in some motor vehicle transmissions of group configuration changes under load between range ratios are enabled by the provision of special connection options of the range group to the group transmission upstream from it.
DE 10 2007 047 671 A1 describes a motor vehicle transmission with a group configuration which consists of a main group in the form of a dual-clutch transmission and a range group connected downstream from the main group. In this case the dual-clutch transmission comprises two partial transmissions with input shafts that extend coaxially with one another, each of which can be connected by way of an associated powershift element to a drive input side of the motor vehicle transmission and by means of a respective transmission ratio step, can be brought into connection with a countershaft of an axially offset countershaft gear arrangement. Furthermore, the input shaft in the form of a central shaft can be coupled in a rotationally fixed manner to a coaxially extending output shaft of the dual-clutch transmission. Between the countershaft and the output shaft four transmission steps are also provided, by means of which in each case rotational movement of the countershaft can be converted into a corresponding rotational speed of the output shaft. Three of the four transmission steps define forward gears of the main group, while one transmission step is made with an intermediate wheel and accordingly, when connected into a force flow, brings about a reversal of the rotation direction of the output shaft compared with the other transmission steps. The range group connected downstream from the main group consists of a planetary stage formed by a planetary carrier, a centrally positioned sun gear and a radially outer ring gear, such that planetary gearwheels guided by the planetary carrier mesh both with the radially inner sun gear and with the ring gear. The sun gear is positioned on the output shaft of the main group, whereas the planetary carrier is connected to a drive output shaft of the range group and thus also of the motor vehicle transmission. To obtain two different range ratios of the range group the radially outer ring gear can on the one hand be fixed to a surrounding housing of the motor vehicle transmission and on the other hand coupled rotationally fixed to the drive output shaft and the planetary carrier.
To be able now to carry out a change of the range ratio even under load, on an axial side facing away from the drive output shaft the planetary carrier of the range group can be connected to a gearwheel of the adjacent gear step of the main group mounted to rotate on the output shaft so that, bypassing the range group, rotational movement of the gearwheel is converted directly into rotational movement of the drive output shaft with a rotational speed corresponding to that of the gearwheel. In this case the change of the range ratio is then brought about in that a highest gear of the main group with the lower range ratio of the range group and a lowest gear of the main group with the higher range ratio of the range group are engaged at the same time by way of one partial transmission of the dual-clutch transmission and correspond to one another in their effective transmission ratios. The lowest gear of the main group with the higher ratio of the range group is obtained by coupling the gearwheel mounted to rotate on the output shaft of the main group to the planetary carrier of the range group, so that the range group operates while free from load and consequently a shift can be carried out within the range group without problems by fixing the ring gear to the housing.